1. Field of the Invention
This invention relates to an uninterruptable power supply (UPS) provided internally to an electronic device and, more particularly, to such a UPS for directly feeding a direct current (DC) voltage generated by rectifying a voltage of a normal power supply to a load without intervention of any voltage converter and, if a disturbance occurs in the normal power supply, for backing up the DC voltage by a power supply such as a battery or the like other than the normal power supply.
2. Description of Related Art
A desktop computer (hereafter simply called "computer") consumes a relatively large amount of power and, thus, it generally uses a commercial power supply of alternating current (AC) as a normal power supply. Many electronic components operating inside the computer, such as processors, memories, disk drives and the like, operate at DC voltages generated by rectifying AC. During operation of the computer, a program and data are temporarily stored into a main memory (RAM) for executing designated tasks. All data created by such work is stored in a main memory but it cannot maintain the stored data when a power loss occurs. Accordingly, in the event that a task performed in the computer is finished before the computer is powered-off, it is necessary to save the data stored in memory into a magnetic disk, a floppy disk or the like. Also, in a case where a number of programs are running, it is desirable to terminate the programs in a predetermined order to stop the computer.
Since DC voltages fed to electronic components in a computer are generated by rectifying AC of the commercial power supply, they may vary due to a power interruption (power failure), an instantaneous voltage drop, a frequency fluctuation or the like. Each electronic component in a computer has a predetermined allowable range (tolerance) for a DC voltage variation respectively to maintain its normal operation. Thus, for normal operation of a computer, it is necessary to continually feed voltages that come within the respective allowable ranges to these electronic components. This is because a sudden occurrence of a voltage or frequency disturbance in the commercial power supply may lead to a malfunction of an electronic component or a loss of working data in a memory, which may result in significant damage to a user's work.
In order to cope with this problem in a computer, a UPS has been customarily provided between the commercial power supply and the computer. Such a UPS is provided with a rechargeable battery, a charger and an inverter for DC/AC conversion. When the commercial power supply is available, it converts AC of the commercial power supply to DC and then, further converts its output to AC for feeding the same to the computer, whereas the charger performs supplementary charge to compensate for natural discharge. If a power interruption occurs, a DC voltage of the battery is converted to AC through the inverter for feeding it to the computer. Since a battery terminal is always connected to an input of the inverter, an output of the inverter remains constant even when feeding power to the inverter is instantaneously switched from the commercial power supply to the battery. The battery requires a minimum capacity that allows the computer to be normally operated for a time period which starts from detection of a power interruption by the computer (or an operator) and ends at completion of an off-sequence for stopping the computer in a predetermined procedure.
In FIG. 1, there is shown a schematic block wiring diagram of a conventional power supply system for a computer using a UPS. More particularly, UPS 14 containing a battery therein is connected to the commercial power supply 12, whereas power supply circuitry of computer 10 is connected to UPS 14. AC100 V generated by UPS 14 is converted to stabilized or regulated DC12 V by a power supply unit 16 provided within the computer 10. A portion of the DC12 V is fed to a DC/DC step-down converter 18 where it is converted to regulated DC5 V and DC3.3 V to be fed to a 5 V/3.3 V load 22, which includes memories and various driver circuits. (For brevity of description herein, the term "load" will be used hereafter in a singular form). Another portion of the DC12 V is fed to a DC/DC step-down converter 20 where it is converted to regulated DC2 V to be fed to a 2 V load 24, which includes a CPU. Yet another portion of the DC12 V is directly fed to a 12 V load 26, including a hard disk drive, liquid crystal display panel and the like, without intervention of any converter. Since the power supply unit 16 is provided with a function for regulating its output voltage, there should be no problem to directly feed the output voltage of the power supply unit 16 to the 12 V load 26 without intervention of any converter. Rather, it would be more efficient to do so because of the absence of a substantial loss in an inverter.
In Japanese Patent Publication No. 9-322433 (Japanese Patent Application No. 8-137879), there is disclosed a power supply system, wherein DC power is fed to a load in parallel at a predetermined ratio from both of a main power supply section and a UPS power supply section, thereby improving efficiency. Upon detection of a malfunction in one of the power supply sections or a power interruption, the other power supply section is rendered to immediately feed DC power, thereby improving reliability. However, a disadvantage with this system is that DC output voltages of the main power supply section and UPS power supply section are interconnected together to guarantee parallel running, and yet a DC/DC converter comprising UPS power supply section is always feeding DC power at a predetermined ratio.
In Japanese Utility Model Publication No. 5-20142 (Japanese Utility Model Application No. 3-67125), there is disclosed a power supply system, wherein both of a circuit for feeding power to a load when the commercial power supply is operatively working and a battery backup circuit for feeding power at the time of a power interruption are provided in parallel to the load at a secondary side of an A/D converter. Further, with respect to the battery backup circuit, a primary battery circuit of a lithium cell or the like and a secondary battery circuit of a capacitor or the like are connected to the load in parallel. Either one of the commercial power supply, primary battery and secondary battery, which should feed power to the load, is determined on the basis of voltage differences among them. However, this arrangement does not use a voltage converter for regulating an output voltage of the A/C converter, which results in a large allowable voltage variation range for the backup element (load), e.g., from 2 V to 5 V. Thus, this arrangement is not suitable for use with a load that has a strict allowable voltage range.
It is not expedient in terms of space for a UPS and cost involved to cope with a sudden variation of the commercial power supply by means of an externally provided UPS as seen in the conventional power supply system for a computer of FIG. 1. Because such a UPS is designed to convert an AC voltage of the commercial power supply to a DC voltage and then convert the same to an AC voltage again, it has a relatively large size and consumes energy during the course of voltage conversions, which renders it costly.
Also, modifying the circuitry of FIG. 1 without using UPS 14 in such a way that a backup power supply, comprising a charger and a rechargeable battery, is provided at a secondary side of the power supply unit 16 and an output of the rechargeable battery is connected to the DC12 V line, then at the time of a stoppage of the commercial power supply, an output voltage of the battery will be directly fed to the 12 V load 26 as well. In this case, a variation range of the output voltage of the battery will deviate from an allowable voltage variation range for the 12 V load 26 and, thus, it will result in a malfunction of the 12 V load 26. Here, in order to regulate the voltage fed from the battery to the 12 V load 26, such a modification may be further modified to provide a 12 V/12 V voltage converter dedicated to the 12 V load 26 in addition to the backup power supply, However, with respect to the 12 V load 26 in the latter modification, power will always be fed from the commercial power supply via the voltage converter and, hence, the latter modification is not expedient in terms of a power loss occurring at the voltage converter. Moreover, it is very difficult to manufacture such a 12 V/12 V voltage converter.